During most surgical procedures the patient is paralyzed and must depend entirely on the anesthesiologist to maintain such basic functions as breathing and heart rate to ensure adequate oxygen delivery to the vital organs of the body. Over the past decade a large number of new monitoring devices have been made available which are primarily directed at preventing catastrophic events such as airway disconnects, anesthetic overdose, and lack of oxygen. There is still little direct and continuous information available to the anesthesiologist about the patient's physiological state such as the measurement of blood gases and cardiac output. At the present time the problem facing the anesthesiologist is that the benefits of cardiac output monitoring must be weighed against the risk of central venous catheterization - the only practical and reliable method currently available for cardiac output measurement. The invasiveness of this method of monitoring cardiac output is not justified for the majority of the surgical patients. However, it is the rare and unsuspected complications in the healthiest patients that constitute the major expense of malpractice claims. This proposal describes the development of a noninvasive cardiac output monitor for use during anesthesia. The monitor will cost less than an anesthesia gas monitor and will prove a substantial monitoring benefit at a disproportionately modest cost for all classes of patients.